Cementation Process. ' I J composition, it may be made either from wrought or cast iron. We shall first consider the former method. Cementation.—In this, the oldest process, bars of wrought iron are placed in fire-clay boxes, Pig. 88, with charcoal dust around and between them, and a layer of fire-clay over all (being the cement giving the name to the process). They are then subjected to a bright-red heat, for a time varying with the amount of carbon required to be introduced, and which may be as much as a fortnight for the more highly carbonised steels. The charcoal then becomes combined with the iron, and the steel so pro- duced is called blister steel, from the fact that the bars are covered with blisters. These bars are next broken up, piled, and heated in a furnace almost exactly like the one in Fig. 85, hammered by rapid blows from a tilt-hammer, Fig. 89, and shear steel of a fibrous quality is thus produced.* Double shear steel is made by breaking in two and re-hammering Crucible cast steel is obtained by melting fragments of blister steel in covered crucibles made of a mixture of fire-clay and plumbago, and placed in sets of six or twelve in furnaces having a similar section to the one shewn in * The steam hammer is used in later-built works. For drawing, see Chapter IV.